In the field of telecommunications, several applications demand the generation of noise in the -70 to -40 dBm0 range. For example, when low level echos on a telephone line are suppressed by a residual suppressor or the like, the line sounds "dead" to the users, who are led believe that the connection has been cut off. Studies have shown that when noise having a power level approximately equal to the background noise level on the near end is injected onto the line, the anxiety of the telephone users is relieved.
In speech compression systems that employ digital speech interpolation techniques, the half-duplex characteristic of voice communication is utilized, so that during periods of silence on a given channel, bandwidth is deallocated. Predictably, the telephone connection associated with the channel will sound disconnected to the users unless low level noise is injected at the far end.
Because most digitized signals in telephony must adhere to a .mu.-law encoding scheme, conventional noise generation schemes must first generate the noise sample via digital signal processing (DSP) techniques and then convert it to an 8-bit pseudo-logarithmic sign-magnitude .mu.-law representation. Details on the definition of .mu.-law may be acquired by consulting Bell Publication 43801.
One traditional method of .mu.-law conversion directly computes from the linear sample generated by DSP. Another method uses an 8K.times.8 look-up table that yields the .mu.-law numeric representation given a linear sample generated by DSP. Both methods require a two step process: 1) compute noise sample in DSP, and 2) convert to .mu.-law representation. Additionally, the first method requires additional hardware and computation power and the second method requires fast memory storage space.
It is therefore desirable to provide for a method and apparatus which generates low level noise in accordance with the .mu.-law encoding scheme without requiring additional processing hardware, time and memory.